The interaction of the bloodplatelet membrane and fibrin is being carried out. This is an interaction required for normal clot retraction. Reports have suggested that a specific receptor for fibrin exists on the platelet membrane surface. We are attempting to verify this as well as expand the study. Various assay procedures for platelet membrane-fibrin binding are being investigated. These include: 1) Adsorbing fibrinogen onto polypropylene tubes, converting the adsorbed fibrinogen to fibrin by thrombin treatment, and examining the binding of I 125 labeled platelets to these surfaces. 2) Examining the binding of I 125 labeled D and E fragments and soluble fibrin complexes to intact platelets. 3) A kinetic study based on the adsorption of fibrinogen to small polypropylene beads. Characterization of the binding will take place as well as attempts to isolate the proposed fibrin receptor. The interaction of fibrin and the platelet membrane will be studied employing chemical crosslinking reagents and immunological methods.